I think there worth it against sm or sm armored/toughness type guys. But its a waste to take a bunch against orks or gaurdsmen. O and it gives the squad with it some anti vehicle, as long as its not a strong vehicle.

I find they work well for me. I use a plasma pistol with my Chaplain and a plasma gun in a tactical squiad. The chaplain is usually assaulting, so he only fires once, it rarely "Gets Hot&#33;", and with his high BS he is almost always going to kill someone. The plasma gun is a little more risky. Some games it never "Gets Hot&#33;", but on average it would get hot once a game. The marine&#39;s armour save usually let&#39;s him live to fire next turn though.

Now, I only use 2 guns and fire them 3 times per turn (approx). So more guns in the army will have a greater chance of misfire. Especially when Rapid Firing.

So I would have to say they are worth it, but used in moderation. 2 to 4 guns would be good if you are worried about killing off your own marines. If not, go with as many as you think.